We will study aldehydes R-CHO and ketones R-CO-R', which contain a carbonyl group >C=O, and carboxylic acids, which contain a carboxyl group -COOH.
Basic Knowledge
Aldehydes: Oxidation products of primary alcohols. They show reducing properties and are detected by the silver mirror reaction(Tollens' reagent) and Fehling's reaction.
① Silver mirror reaction: R-CHO + 2[Ag(NH3)2]+ + 3OH- → R-COO- + 2Ag↓ + 4NH3 + 2H2O. A silver mirror forms on the container wall.
② Fehling's reaction: The aldehyde reduces Cu2+(blue) to Cu2O(a brick-red precipitate).
Main aldehydes: formaldehyde HCHO(formalin, a 37% aqueous solution, used for preservation and specimens), acetaldehyde CH3CHO(an intermediate in alcohol metabolism and a cause of hangovers), benzaldehyde C6H5CHO(aromatic, covered in a later course).
Ketones: Oxidation products of secondary alcohols. Unlike aldehydes, they have no reducing properties(negative silver mirror and Fehling's reactions). A representative example is acetone (CH3)2CO(solvent, positive iodoform reaction).
Carboxylic acids: -COOH. They are weak acids(Ka≈10-5), stronger than carbonic acid, and release CO2 from NaHCO3(a detection reaction).
Main carboxylic acids: formic acid HCOOH(ants and bee venom, the only one that also shows reducing properties and acts as an aldehyde), acetic acid CH3COOH(vinegar, glacial acetic acid with a melting point of 16.7℃), oxalic acid (COOH)2(dihydric, reducing agent, standard substance for titration), lactic acid CH3CH(OH)COOH(muscle fatigue and yogurt), citric acid(lemons, trihydric), maleic acid/fumaric acid(geometric isomers of a C=C double bond).
Aldehyde R-CHO(reducing properties, positive silver mirror and Fehling's reactions)
Ketone R-CO-R'(no reducing properties, positive iodoform reaction for methyl ketones)
Silver mirror reaction(silver deposition with Tollens' reagent)
Fehling's reaction(Cu2+ blue→Cu2O brick-red precipitate)
Dimerization of carboxylic acids(dimer formation by hydrogen bonding in the gas phase, apparent molecular mass doubled)
Fatty acids(long-chain carboxylic acids, such as stearic acid C17H35COOH)
Further Exploration (Principles and Applications)
Formaldehyde HCHO is the smallest aldehyde. As formalin, it is used to preserve biological specimens, as a raw material for Bakelite(phenolic resin), and in adhesives for MDF plywood. It is a major cause of sick building syndrome, and its use is now strictly regulated. Its carcinogenicity has been confirmed(IARC Group 1).
Acetaldehyde is an intermediate in ethanol metabolism: C2H5OH → CH3CHO → CH3COOH(alcohol dehydrogenase ADH→aldehyde dehydrogenase ALDH2). About 40% of Japanese people have low ALDH2 activity, so acetaldehyde accumulates and causes facial flushing, headaches, and nausea(a constitution that is sensitive to alcohol). This is a genetic polymorphism characteristic of Asians and an important topic in toxicology.
Carboxylic acids are important in both industry and life science: fatty acids(raw materials for soap and detergents), amino acids(proteins), the citric acid cycle(cellular energy metabolism), aspirin(acetylsalicylic acid), and so on.
- Aldehydes=have reducing properties(silver mirror and Fehling's reactions)
- Ketones=have no reducing properties
- Formalin=37% aqueous solution of HCHO
- Acetaldehyde is an intermediate in alcohol metabolism
- Carboxylic acids are weak acids and release CO2 from NaHCO3
- Formic acid HCOOH is the only carboxylic acid that shows reducing properties(it also has an aldehyde structure)
- Carboxylic acids form dimers in the gas phase
Points to Note(Common Confusions and Frequent Mistakes)
① Formic acid HCOOH also has a -CHO structure, so it gives positive silver mirror and Fehling's reactions(other carboxylic acids are negative). ② Acetone gives a positive iodoform reaction(CH3-CO- structure). ③ Acid strength: carboxylic acids>carbonic acid>phenol(therefore carboxylic acids release CO2 from NaHCO3, while phenol does not react with NaHCO3 but reacts with Na2CO3). ④ Maleic acid readily forms maleic anhydride by dehydration because it is the cis form(intramolecular cyclization), whereas fumaric acid cannot.
Practice
- Write the equation for the silver mirror reaction of acetaldehyde.
- Explain, from the viewpoint of structure, why formic acid gives a silver mirror reaction unlike other carboxylic acids.
- Describe two chemical methods for distinguishing acetone from acetaldehyde.